This invention relates to a method for preparing high wet-strength green bodies of cordierite-forming admixtures, to a method for preparing cordierite substrates having high isostatic strength and to the admixtures of minerals and processing aids utilized to prepare such high wet-strength green bodies and cordierite substrates.
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of gaseous emissions from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in model year 1975.
Catalytic converters often comprise cordierite substrates, onto which are coated noble metal catalysts. The cordierite substrates are typically in the form of a honeycomb body or multichannel monolith having substantially parallel cells or passages extending between open end faces thereof. The converter is placed in the path of the exhaust effluent of an automobile engine where the catalysts may act to convert hydrocarbons, CO and NOx to the non-toxic by-products water, carbon dioxide and reduced nitrogen species.
Those skilled in the art know that a sintered cordierite structure is typically made by mixing raw materials such as talc, kaolin clay, alumina, silica, aluminum hyroxide, various inorganic and organic binders and water to form a batch composition. See, for example, the disclosure of U.S. Pat. No. 3,885,977. This composition is then extruded into a honeycomb body. At this stage, before sintering, the honeycomb body is still wet and is frequently referred to as being "green". The term "green" is used in the art and in this application to refer to the state of a formed body or piece made of sinterable powder or particulate material that has not yet been fired to the sintered state. Only after firing at high temperatures does the structure obtain the strength and other characteristics of the final, desired ceramic product.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. At least in part because of a desire to lower exhaust backpressure in vehicles and thereby increase engine power, there has been a move to producing thinner-walled cordierite monoliths or honeycombs for use as catalyst carriers. It has also been desired to optimize the thermal shock resistance and strength of the cordierite substrates. It has been suggested, in U.S. Pat. No. 4,772,580, that these properties of cordierite structures can be enhanced by utilizing fine particles of both talc and kaolin in the preparation of
the cordierite. U.S. Pat. No. 4,772,580 suggests that the talc particles should have an average particle diameter of not greater than 7 .mu.m and the kaolin particles should have an average particle size of not greater than 2 .mu.m and not greater than 1/3 of the average particle diameter of the talc particles and other raw materials for producing the cordierite.
Although the use of such fine raw materials leads to improved sintered products, it also results in additional processing difficulties. It is important that the green honeycomb body, prior to firing, have good strength, termed wet strength, as some handling of the body is inevitable subsequent to extrusion and prior to firing. For example, if a distortion is introduced at the skin region extruded green body while it is being transferred to the drying process, it may cause structural imperfections in the body which will reduce the strength (e.g., the isostatic strength) of the final, sintered ceramic product. Unfortunately, experience has shown that the wet strength of green bodies prepared from fine raw materials is not as great as that of such bodies when coarser materials are used. As a result, it is more likely that imperfections will be introduced in the body during handling and that the final sintered products will have unacceptably low isostatic strength.
It must also be noted that, to manufacture thinner wall substrates, the slots in dies through which the batch composition will be extruded must necessarily be thinner. Such thin slots have very high impedances, resulting in significantly elevated pressures and torques. One might add more water to the batch composition to lower pressures and torques, but this would also be expected to further reduce the wet strength of the extruded green body.
There is, accordingly, a clear need for a means for preparing sintered cordierite structures which exhibit the thermal shock resistance properties of products currently made from fine raw materials, but which are more easily processed, which exhibit good wet strength in the green state and which, as a result, exhibit good isostatic strength in the final sintered state.